Vauxhall Zafira 53 Plate Dash lights go out when I switch on headlights?

Hi can anyone help my Daughters 53 plate Vauxhall Zafira electrical fault.
Her husband has just fitted a replacement Drivars side mirror assembly works fine but now when she puts the headlights on the dash lights go out she says it was fine before ?. Can't see that the mirror would make this happen could there be another fault causing this.
Any pointers would be appreciated